The present invention relates to scroll compressors having swing-link radial compliance drive mechanisms. More specifically, the field of the invention is that of bearing arrangements between a crankshaft crankpin and a pivoting roller of the swing-link drive mechanism.
One example of a scroll compressor is found in U.S. Pat. No. 4,875,838, assigned to the assignee of the present invention, the disclosure of which is expressly incorporated by reference. In such a scroll compressor, a movable scroll wrap is disposed within a fixed scroll wrap, and a swing-link drive mechanism translates orbiting and rotating motion from an eccentric crankpin on the end of a crankshaft to an orbiting motion of the movable scroll wrap within the fixed scroll wrap. The orbiting scroll wrap is prevented from rotating about its own axis by a conventional Oldham ring assembly.
During operation of a scroll-type compressor, the pressure of compressed refrigerant at the interface between the scroll members tends to force the scroll members axially and radially apart, thereby permitting high to-low pressure leakage between compression pockets that reduces the operating efficiency of the compressor. Consequently, axial and radial compliance of the orbiting scroll member toward the fixed scroll member is required in order to maintain the scroll members in sealing contact with one another. Several methods for achieving axial and radial compliance have been developed, and are widely used in scroll-type compressors.
One method of achieving radial compliance in a scroll-type compressor is to utilize a swing-link radial compliance drive mechanism of the type disclosed in U.S. Pat. No. 4,875,838. Generally, the swing-link drive mechanism includes a roller pivotally journalled about an eccentric crankpin for imparting orbiting motion to the movable scroll wrap and, at the same time, causing the movable scroll wrap to radially comply with the fixed scroll wrap. Specifically, the eccentric crankpin on the crankshaft is received within an eccentric axial bore of a cylindrical roller, whereby the roller is eccentrically journalled about the eccentric crankpin. The roller and crankpin assembly is then received within a cylindrical well formed on the bottom surface of the orbiting scroll wrap, whereby rotation of the crankshaft causes the orbiting scroll wrap to orbit.
Ideally, the fixed and orbiting scroll wraps would have perfectly matching and perfectly smooth surfaces and, consequently, the swing-link mechanism would operate smoothly, i.e., the roller would experience no movement relative to the crankpin or the roller would pivot smoothly to promote sealing engagement between the involute scroll wraps. In either case, a sufficient oil film would be maintained between the crankpin and roller to minimize wear. However, in practical situations the wraps contain minor imperfections that adversely affect the operation of the swing-link mechanism.
Imperfections in the geometry and/or surface finish of the wraps result in perturbations during operation of the swing-link mechanism, which cause chattering of the roller relative to the crankpin. Chattering vibrations result in rapid back and forth movement of the roller relative to the crankpin with an extremely small, almost microscopic, displacement. Meanwhile, a driving force is focused in the direction of a line that is tangential to the orbiting motion of the crankpin and roller, with the force being applied at a line of contact on the circumference of the crankpin onto the roller. The chattering is so small that an oil film cannot be established between the crankpin and roller surfaces, and so fretting occurs. Fretting is a condition where the molecular bonds between the materials at the line of contact are broken down and very severe and localized wear occurs. The chattering vibrations are totally random, and depend on the perturbations which occur as the fixed and orbiting scroll wraps engage.
Another type of radial compliance mechanism used in a prior art scroll compressor includes a cylindrical unloading drive bushing having an axial bore in which is drivingly disposed an eccentric crankpin. The axial bore is slightly oval in cross-section, and includes a flat bearing insert disposed in the wall of the bore. A flat on the crankpin slidably engages the flat bearing insert as the crankpin drives the brushing. The oval cross-section of the bore permits limited sliding movement between the crankpin and bushing, in order to achieve radial compliance and unloading.
However, a problem inherent with the aforementioned radial compliance mechanism is that vibrations from a change in load tends to cause noise and premature wear of the crankpin and roller. When the compressor starts-up, for example, the loose-fitting crankpin may shake and cause noise, or when the compressor changes speeds the loose-fitting crankpin may shift position slightly and rattle. In either case, the additional noise is undesirable and the additional wear should be avoided.
What is needed is a radial compliance mechanism that minimizes the problems associated with chattering vibrations between the crankpin and roller.
Also needed is a radial compliance mechanism that can be incorporated into existing compressor designs.
A further need is for a radial compliance mechanism that is relatively easy to manufacture.